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Thursday, August 14, 2008

Hearing What’s Not There

Sometimes, making data disappear can be acceptable

Ever wonder how magicians make a large object disappear, or a woman’s dress instantly change color?  According to a study in Nature Reviews Neuroscience, cognitive scientists have been wondering as well. The scholarly, footnoted article explains magic tricks in terms of the visual and neurological quirks they rely on. It credits The Amazing Randi, The Great Tomsoni, and Teller (of “Penn and...") as co-authors. It’s visually oriented - as is a lot of magic - but abracadabra: Here on the audio side, we’ve been benefitting from that kind of research for years. You can benefit, too.

The article makes no bones about magic being primarily visual:

Much as early filmmakers experimented with editing techniques to determine which technique would communicate their intent most effectively, magicians have explored the techniques that most effectively divert attention or exploit the shortcomings of human vision...

But this is an audio blog, so I’ll cover equivalent audio techniques: How we use neurological and mathematical tricks to make data in an audio file disappear… without noticeably affecting the sound.

In other words, it’s about getting the most from mp3, AAC, and other techniques. This often means not doing what the audio programs’ menus seem to suggest. If you’ve got anything to do with moving audio on the Internet, these tricks will be helpful. Some of them also apply to movies and broadcast. (Did you know that Dolby theatrical releases are much more compressed than most 99 ¢ music downloads?)

It’s a big topic, and I don’t want to get ahead of myself. So I’m releasing this article in four parts over the next ten days or so:

• What’s going on inside your head when you hear things


• How psychoacoustic compression algorithms work


• How lossless compression shrinks an audio file without any changes… but only if you’ve got a powerful computer


• A few non-intuitive tricks that can streamline how you and your clients exhange audio over QuickTime.
  

On to part one.

Audio • Post Production •

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hey Jay

great article, again…

got a question…
can you fill us in on the “calibrated to theatrical levels”?
for example..  . . i mix the explosions to 0db ( just at clipping)
what is a good level for dialog , then? -30 db?
i have heard that dialog should be around 85-90 db in the theatre, and the big boom-booms go to 120 db
any tips?
THANKS !!

i have mainly done music and commercials, and you know they want that stuff at the bare edge of clipping all the time… but we all know nobody can stand that for 90 minutes in a theatre....

Posted by billS  on  08/18  at  04:35 AM

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Glad you liked it, Bill.

Full size mix theaters are calibrated so that -20 dBFS on the track = 85 dB SPL, at the listening position, from a single speaker. (Smaller rooms are often set to -82 dB SPL instead.)

That puts normal dialog around -20 dBFS in the center channel.  But of course it’s really up to how the scene feels (which is why the theater is calibrated).

Of course of course, it’s really up to the director.

That also puts the 0 dBFS boom booms at 105 dB SPL… per speaker. How the speakers will add depends on what’s in each channel.

If you played something through all 6 channels in phase, you could reach 120 dB SPL. Boy, would I hate to sit in that theater.

(Further, that assumes the theater when it’s showing is calibrated. Lots of luck there...)

Posted by JayR  on  08/18  at  10:16 AM

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